1. Field of the Invention
The present invention relates to active matrix substrates that are used in display devices such as liquid crystal display devices.
2. Description of the Related Art
FIG. 15 indicates a conventional arrangement of an active matrix substrate used in liquid crystal display devices. As indicated in FIG. 15, an active matrix substrate 100 includes (i) a plurality of scanning signal lines 116, (ii) a plurality of data signal lines 115, arranged to cross each other, (iii) TFTs (Thin Film Transistors) 112 that are formed in the vicinity of intersection points of both signal lines (115 and 116), and (iv) pixel electrodes 117. A source electrode 119 of the TFT 112 is connected with the data signal line 115, and a drain electrode 108 is connected with the pixel electrode 117 via a drain lead electrode 107. The scanning signal line 116 also works as a gate electrode of the TFT 112.
A hole is created in an insulating film disposed between the drain lead electrode 107 and the pixel electrode 117, which hole provides a contact hole 110 that connects the drain lead electrode 107 and the pixel electrode 117. The pixel electrode 117 is a transparent electrode made of ITO or the like, which light (backlight) from beneath the active matrix substrate can pass through.
On this active matrix substrate 100, a scanning signal (gate ON voltage) sent through the scanning signal line 116 turns on the TFT 112 (that is, the TFT 112 is put in a state of allowing a current flow from the source electrode 119 to the drain electrode 108). When the TFT is ON, a data signal (signal voltage) sent through the data signal line 115 is written on the pixel electrode 117 via the source electrode 119, the drain electrode 108 and the drain lead electrode 107. A storage capacitor (Cs) wiring 118 prevents self-discharge of a liquid crystal layer while the TFT 112 is off.
In the manufacturing process of this active matrix substrate 100, foreign objects, film residues and the like may cause a short circuit or a leak between the source electrode 119 and the drain electrode 108. If such a TFT defect occurs, a normal voltage or drain voltage is not applied to the pixel electrode 117, which in turn causes a pixel defect, such as a bright dot and dark dot. This decreases the production yield of liquid crystal display devices.
As a method of dealing with such TFT defects, a redundant structure in which a plurality of TFTs are connected in parallel for one pixel has been proposed in Japanese Unexamined Patent Application Publication No. 7-199221, for example.
The liquid crystal display device set forth in Japanese Unexamined Patent Application Publication No. 7-199221 provides redundancy by arranging a plurality of TFTs (active elements) in parallel. Such a redundant structure increases parasitic capacitance between the TFTs and the scanning signal lines, and leads to lessened display quality (particularly in displaying moving images) as signal write frequencies have become higher in recent years. In addition, the electric power consumption increases as capacity loads grow larger, and the aperture ratio decreases because a plurality of TFTs are arranged in parallel.